The present invention relates generally to the Stretford process for removing a hydrogen sulfide constituent from a gas stream and recovering elemental sulfur as a by-product. More particularly, the present invention concerns an improvement to the gas/liquid contacting step by which the Stretford solution is exposed to the gas stream for reaction with the hydrogen sulfide constituent.
Hydrogen sulfide is a noxious gas which is commonly found in considerable concentrations in sour natural gas and in tail gases from petroleum refineries. The noxiousness of hydrogen sulfide is manifested in a number of ways. For example, an offensive odor is detectable when hydrogen sulfide is present in quantities as low as 0.13 ppm by volume. In addition, a mixture of hydrogen sulfide and air is explosive when the hydrogen sulfide is present in concentrations as low as 4.4 volume percent. Moreover, hydrogen sulfide is a dangerous mammalian poison.
The noxious character of hydrogen sulfide has led to state and federal laws and regulations that severely restrict the quantities of hydrogen sulfide which may be permissibly exhausted into the atmosphere. In at least partial response to these regulations, numerous processes have been developed to remove hydrogen sulfide from residue gases as well as from otherwise useful products such as natural gas. One of the prominent processes in the petroleum industry for effecting the hydrogen sulfide removal is known as the Stretford process. The Stretford process is generally described in three United States patents issued to T. Nicklin et al, U.S. Pat. No. 2,997,439 issued Aug. 21, 1961; U.S. Pat. No. 3,035,889 issued May 22, 1962; and U.S. Pat. No. 3,097,926 issued July 16, 1963.
According to the Stretford process, gaseous hydrogen sulfide is reacted with a solution containing anthraquinone disulphonic acid and an aqueous alkaline solution containing ortho-, meta- and pyro-vanadates of ammonia and alkali metals and a salt of iron, copper, manganese, chromium, nickel, or cobalt. The gaseous hydrogen sulfide, which may be only one constituent of a mixture of gases, is then exposed to the Stretford solution where the reaction occurs. In the reaction, hydrogen sulfide is oxidized by the Stretford solution to elemental sulfur and the solution is reduced. Then air, or an oxygen containing gas, bubbles through the reduced Stretford solution to regenerate that solution by oxidizing it. Subsequently, sulfur is separated from the Stretford solution and the solution is recirculated in the process. Regeneration of the Stretford solution and separation of the elemental sulfur from the Stretford solution typically take place with a flotation cell.
In the typical Stretford process apparatus, the gas/liquid contacting step occurs in a scrubber using a venturi section. As the gas containing hydrogen sulfide passes through the venturi, the Stretford solution is sprayed into the gas stream in the form of fine droplets. The chemical reaction discussed above occurs across the surface area of these droplets as they flow downstream along with the gas flow. Generally, the gas-liquid contact occurs with the gas phase and liquid phase moving, in a co-flowing relationship. Thus, a multi-phase flow of gas, liquid are solid particles (sulfur) leaves the venturi discharge.
In conventional Stretford process systems, the multi-phase flow from the venturi moves vertically downwardly and passes through a conduit that first turns the flow to a generally horizontal direction and then discharges the flow into the bottom of an absorbing column. The liquid and solid phases of the multi-phase flow are generally separated by centrifugal and gravitational forces as the gas phase decelerates into the large volume absorber column and turns to move vertically upwardly through the absorber column. A second gas/liquid contact occurs in the absorbing column where additional Stretford solution passes in counterflow relationship with the gas stream through the absorber column media and drops to the bottom of the absorbing column.
It has also been known to provide a short vertically extending dam in the bottom of the horizontal portion of the conduit between the venturi discharge and the absorber column in a Stretford process. This short dam has been employed for the purpose of retaining a shallow pool of the Stretford solution which backs up toward the elbow provided in the conduit. This retained pool of Stretford solution provides some protection to the elbow against the corrosive and erosive effects of the liquid, gas and solid particles passing therethrough.
In other processes and apparatus for gas scrubbing, it is known to provide baffles for generating turbulence in a gas flow into which liquid is sprayed. U.S. Pat. No. 4,334,897, issued June 15, 1982 to Brady et al (sulphur dioxide). In other gas scrubbers baffles direct a gas stream as it moves across the surface of a liquid reservoir. U.S. Pat. No. 2,256,374, issued Sept. 16, 1941 to Cummings, Jr. (air humidified by water); U.S. Pat. No. 3,172,725 issued Mar. 9, 1965 to Rugh (sulfur trioxide removed from air by sulfuric acid solution); U.S. Pat. No. 3,815,332 issued June 11, 1974 to Babrowsky et al (solids removed from gas by water); and U.S. Pat. No. 4,005,999 issued Feb. 1, 1977 to Carlson (particulate pollutant removed from industrial process effluent by a cleansing liquid).
It is preferred that primary gas/liquid contct occur upstream of the absorber column. This result is desirable because the small passages of the absorber column become clogged if large quantities of elemental sulfur occur therein. Generally speaking, as much as 95% to 98% of the sulfur should be removed from the gas before it enters the bottom of the absorbing column. This high level of sulfur removed in the venturi scrubber is needed since the elemental sulfur agglomerates in particles having sizes ranging from submicron dimensions to 100 microns. Particles of such size could plug the packing in the absorber column but, due to the openness of the venturi and downstream conduit, these can be easily handled by the venturi scrubber.
When the Stretford process is scaled up for large plant capacities, the venturi of the scrubber may require a throat diameter in excess of two feet and a very large flow rate of Stretford solution. It has been observed that inadequate gas/liquid contact occurs in the scrubber and venturi section in large sized venturis. Moreover, it has been observed that with a large venturi and a high liquid mass flow rate, as little as 65% of the desired sulfur removal occurs in the scrubbing step. While various theories have been advanced to explain this deficiency, it is clear that the need continues to exist for an improvement to the Stretford process which overcomes these problems.